1. Magnetic Materials & Data Storage Research Laboratory, Faculty of Engineering, Khon Kaen University, Khon Kaen 40002, Thailand
2. KKU-Seagate Cooperation & EMI/EMC Research Laboratory, Faculty of Engineering, Khon Kaen University, Khon Kaen 40002, Thailand
3. Faculty of Applied Science and Engineering, Khon Kaen University, Nong Khai Campus, Nong Khai 43000, Thailand